A typical prior art patent is U.S. Pat. No. 3,703,718 which discloses an optical collecting means for use in an infrared intrusion detector system. FIG. 1 of the present invention discloses a segmented spherical mirror system of the type disclosed in that patent.
Some of the previous problems associated with optical systems using detection of radiated energy, whether visible or invisible, stem from a physical limitation; that to achieve a sharp focus the mirror or lens system has to be of a relatively small size. Accordingly, known optical systems have a correspondingly small collecting area and generally only a small portion of the image can be utilized by a typical sensing element.
As an example of a system of the type disclosed in U.S. Pat. No. 3,703,718, consider an object, such as a person, having a length of two meters positioned at a distance of 10 meters from the detector. Assuming ideal focusing, the image is at or very near the focal point of the mirror. The image length is represented by the following equation: EQU I = V/U (X).
where I equals image size:
X = object size
U = distance of object from detector
V = distance of image.
Assuming that the distance V is equal to the focal length and that the focal point is at a distance of 10 centimeters (cm) then the image length is calculated as follows: ##EQU1## Thus, in the above example the image has a length of approximately 2 centimeters. However, a typical sensor covers a length of only 0.2 centimeters and thus only about one-tenth of the image is available for generating a signal at the detector. Larger sensors can be constructed, but are impractical as they become excessively costly.
Another problem associated with using an optical system of the type disclosed in U.S. Pat. No. 3,703,718, is concerned with the orientation of the image with respect to the detector. The detectors are normally mounted in an enclosure as taught by that patent with a suitable window. As a result, and particularly because of the concave mirror system, not all of the radiation reaches the sensing element and thus the efficiency of the system is impaired.
However, in accordance with the present invention, primarily only parallel rays are collected and thus the image is sharply focused.
Other problems relating to these prior art infrared detection systems pertain in particular to the detection circuitry.
Difficulties arise when it is attempted to detect low level signals using detectors which have a comparatively high noise level. The sensors themselves are usually high impedance devices such as a thermistor. Impedance matching networks are therefore required and because of the low-level signal high gain amplifiers are also required. There is an attendant tendency in these circuits for a low frequency component to be present which at the output, constitutes a not negligible portion of the signal level. A typical system would use a filter network.
However, in accordance with the present invention and to reduce the need to filter this low-level frequency signal, and also to supervise the operation of the sensing element, a modulation technique is used which greatly simplifies the filtering operation and level detection of the signal.
Accordingly, one important object of the present invention is to provide an improved optical system for an infrared intrusion detection system and that is characterized by an increase in the amount of radiation that is collected.
A further object of the present invention is to an optical collecting system for gathering parallel rays from the object which are collected at a point focus.
Another object of the present invention is to provide an optical system in accordance with the preceding object and that is relatively simple in construction, easily constructed, readily adjustable, and can be manufactured at reasonable cost.
Another important object of the present invention is to provide improved detection and supervision circuitry for use in an infrared detection system.